A Variational Principle in the Dual Pair of Reproducing Kernel Hilbert Spaces and an Application

Given a positive definite, bounded linear operator A on the Hilbert space H0 := l(E), we consider a reproducing kernel Hilbert space H+ with a reproducing kernel A(x, y). Here E is any countable set and A(x, y), x, y ∈ E, is the representation of A w.r.t. the usual basis of H0. Imposing further conditions on the operator A, we also consider another reproducing kernel Hilbert space H − with a kernel function B(x, y), which is the representation of the inverse of A in a sense, so that H − ⊃ H0 ⊃ H+ becomes a rigged Hilbert space. We investigate a relationship between the ratios of determinants of some partial matrices related to A and B and the suitable projections in H − and H+. We also get a variational principle on the limit ratios of these values. We apply this relation to show the Gibbsianness of the determinantal point process (or fermion point process) defined by the operator A(I + A) on the set E. It turns out that the class of determinantal point processes that can be recognized as Gibbs measures for suitable interactions is much bigger than that obtained by Shirai and Takahashi.